1. Technical Field
This invention relates to inductively coupled plasma reactors used in the manufacture of products such as microelectronic semiconductor devices, and more particularly, to an apparatus and method for controlling the radio frequency (RF) induction field over the surface of the workpiecer being processed in such a reactor so as to produce a uniform etch or deposition rate.
2. Background Art
Inductively coupled plasma reactors for processing microelectronic semiconductor wafers are employed where high density plasmas are desired. For example, inductively coupled plasma reactors can produce plasma densities on the order of 10.sup.11 ions/cm.sup.3. Referring to FIGS. 1-4, an inductively coupled reactor generally includes a reactor enclosure defining a processing chamber 12, a pedestal 14 within the chamber 12 for supporting a wafer 16 during processing, a gas inlet system 18 for admitting a gas into the chamber 12, a vacuum pump 20 to evacuate the chamber 12 to a desired pressure, an antenna disposed around or near a portion of the reactor chamber overlying the wafer, and a RF power source 22 connected to the antenna through an impedance matching network 24. The reactor may also include a second RF power source 26 connected to the pedestal 14 through a second impedance matching network 28. The second source 26 is used to create a bias potential on the wafer 16 which is beneficial in some wafer processing operations.
One type of inductively coupled plasma reactor is characterized by a flat coil antenna 30, as shown in FIG. 1. This antenna 30 is typically disposed in or adjacent to a flat top 32 of the reactor and lies in a plane generally parallel to the surface of the wafer 16 being processed. Another type of inductively coupled plasma reactor has a cylindrical antenna 34 disposed in or around a cylindrical sidewall 36 of the reactor, as shown in FIG. 2. Yet another type of inductively coupled plasma reactor has a dome-shaped antenna 38 disposed in or adjacent to a dome shape top 40 to the reactor, as shown in FIG. 3. Some inductively coupled plasma reactors even have combined antenna structures. For example, FIG. 4 depicts a reactor having an antenna 42 with a dome-shaped upper portion and a cylindrical bottom portion.
Inductively coupled plasma reactors are used for a variety wafer processing operations, such as etching, chemical vapor deposition, and so forth. These operations typically require a uniform etch rate or deposition rate across the entire surface of the wafer being processed. For example, non-uniform etching can result in damage to the microelectronic devices being formed on the wafer. One common cause of this damage is referred to as punchthrough. Punchthrough occurs when a portion of the wafer's surface is etched at a higher rate than the remaining surface. In order to ensure complete removal of the material being etched from the center portion, the etching process must be continued beyond the point where this material has been completely removed in a region of the wafer where the etch rate is higher. Punchthrough results from the unwanted etching of the underlying material layers in this region. Likewise, problems can arise in the absence of a uniform deposition rate. For example, depending on when the deposition process is terminated, a non-uniform deposition rate could result in areas of the wafer having a deposition layer which is either too thin or too thick in comparison with a desired layer thickness.
A uniform etch rate or deposition rate can be achieved by making the plasma density as uniform as possible in the region overlying the wafer. For the most part, a uniform plasma density is achieved by using the reactor's antenna to maintain an RF induction field in the chamber which produces a consistent ion current flux across the surface of the wafer (assuming a uniform distribution of processing gas within the chamber). Maintaining such an RF induction field has presented a problem when using currently existing inductively coupled plasma reactor configurations. For example, a reactor employing a flat coil antenna 30 (as shown in FIG. 1) can exhibit a higher RF induction field near the center of the wafer 16 than at the wafer's periphery. Generally, a higher RF induction field level in a region of the chamber will create a higher ion current flux in that region. Accordingly, a higher RF induction field level created over the center of the wafer 16 by a flat coil antenna 30 produces a high ion density or "hot spot" in the plasma over that area, while a lower ion density is exhibited at the wafer's periphery. Thus, the etch rate or deposition rate can be non-uniform across the wafer's surface. Even reactors having dome-shaped antennas (to ameliorate the non-uniform ion density problem) exhibit a non-uniform ion current flux, and therefore a non-uniform ion density, across the surface of the wafer. Likewise, reactors having combined antenna structures also tend to exhibit some degree of non-uniformity in the ion density over the wafer. For example, in the case of a reactor employing a combined antenna having a dome-shaped upper portion and a cylindrical lower portion, RF power from the cylindrical portion of the antenna is absorbed by the plasma near the chamber sidewall. This results in a higher ion density at the periphery of the wafer than at its center. Here again, the non-uniformity in ion density can result in an uneven etching or deposition over the surface of the wafer.
Varying reactor operating parameters, such as chamber pressure, processing gas composition, and RF power levels can also impact the ion current flux profile within the chamber for a given antenna configuration. Thus, one set of operating parameters might produce a desired uniformity in etching or deposition, whereas a different set of parameters may not. In view of this, a reactor may be limited to operating under the particular parameters which produce a substantially uniform etching or deposition rate. Such a limitation unduly restricts the reactor's processing capability.
Accordingly, there is a need to control the RF induction field over the surface of the wafer in such a way as to produce a uniform ion current flux (and so a uniform etch or deposition rate) in a variety of inductively coupled plasma reactor configurations and for a large range of operating parameters.